arXiv subjects

H. C. Ferguson

Publications and source records attributed to H. C. Ferguson.

At least 109 records · Page 6Linked to original sources

arXiv2003

Strong Gravitational Lens Candidates in the GOODS ACS Fields

We present results from a systematic search for strong gravitational lenses in the GOODS ACS data. The search technique involves creating a sample of likely lensing galaxies, which we define as massive early-type galaxies in a redshift range 0.3 < z <1.3. The target galaxies are selected by color and magnitude, giving a sample of 1092 galaxies. For each galaxy in the sample, we subtract a smooth description of the galaxy light from the z_{850}-band data. The residuals are examined, along with true-color images created from the B_{435}V_{606}i_{775} data, for morphologies indicative of strong lensing. We present our six most promising lens candidates, as well as our full list of candidates.

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arXiv2003

Color-selected galaxies at z~6 in the Great Observatories Origins Deep Survey

We report early results on galaxies at z~6, selected from Hubble Space Telescope imaging for the Great Observatories Origins Deep Survey. Spectroscopy of one object with the Advanced Camera for Surveys grism and from the Keck and VLT observatories a shows a strong continuum break and asymmetric line emission, identified as Lya at z=5.83. We detect only five spatially extended, z~6 candidates with signal-to-noise ratios > 10, two of which have spectroscopic confirmation. This is many fewer than would be expected if galaxies at z=6 had the same luminosity function as those at z=3. There are many fainter candidates, but we expect substantial contamination from foreground interlopers and spurious detections. Our best estimates favor a z=6 galaxy population with fainter luminosities, higher space density, and similar co-moving ultraviolet emissivity to that at z=3, but this depends critically on counts at fluxes fainter than those reliably probed by the current data.

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arXiv2003

The Star-Forming Dwarf Galaxy Populations of two z ~ 0.4 Clusters: MS1512.4+3647 and Abell 851

We present the results of a deep narrow-band [OII] 3727 Åemission-line search for faint ($g <$ 27), star-forming galaxies in the field of the $z=0.37$ MS1512.4+3647 cluster. We find no evidence for an over-density of emission-line sources relative to the field at $z \sim$ 0.4 (Hogg et al. 1998), and therefore conclude that the MS1512.4+3647 sample is dominated by field [OII] emission-line galaxies which lie along the $\sim$ 180 Mpc line of sight immediately in front and behind the cluster. This is surprising, given that the previously surveyed $z=0.41$ cluster Abell 851 has 3-4 times the field emission-line galaxy density (Martin et al. 2000). We find that the MS1512.4+3647 sample is deficient in galaxies with intermediate colors (1.0 $< g-i <$ 2.0) and implied star-formation exponential decay timescales $τ\sim$ 100 Myr - 1 Gyr that dominate the Abell 851 emission-line galaxy population. Instead, the majority of [OII] emission-line galaxies surrounding the MS1512.4+3647 cluster are blue ($g-i \leq 1.0$) and forming stars in bursts with $τ< $ 100 Myr. In both samples, galaxies with the shortest star-formation timescales are preferentially among the faintest star-forming objects. Their i luminosities are consistent with young stellar populations $\sim 10^8 - 10^9 \Msun$, although an additional factor of ten in stellar mass could be hiding in underlying old stellar populations. We discuss the implications for the star-formation histories of dwarf galaxies in the field and rich clusters.

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arXiv2002

Observational Properties of Primordial Stellar Populations

We present the first results of a study of the expected properties of the first stellar generations in the Universe. In particular, we consider and discuss a series of properties that, on the basis of the emission from associated HII regions, permit one to discern {\it bona fide} primeval stellar generations from the ones formed after pollution from supernova explosions. The expected performance of NGST for the study and the characterization of primordial sources is also discussed.

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arXiv2000

The nuclear cusp slopes of dwarf elliptical galaxies

We derive the light profiles for a sample of 25 dwarf elliptical galaxies observed by us with HST/WFPC2 in F555W and F814W. These profiles are fitted with Nuker, R^1/4, exponential, and Sersic laws, and are also used to derive the nuclear cusp slopes gamma. We discuss the correlation of nuclear cusp slope with galactic luminosity, presence of a nucleus, and type of light profile. The results are compared to those found in the literature for elliptical galaxies and the bulges of spiral galaxies. We find that as a class the nuclear regions of dwarf ellipticals are very similar to those of the exponential bulges of spiral galaxies, and have nuclear cusp slopes shallower than those of bulges that were well fitted by a de Vaucouleurs R^1/4 profile with the same luminosity. For the 14 nucleated galaxies in our sample this conclusion is less certain than for the 11 non-nucleated objects since it relies on an extrapolation of galaxy light under the nucleus. In terms of their light profiles and nuclear properties, most spheroidal stellar systems can be broadly divided into two subclasses: the exponential shallow cusp objects and the R^1/4, steep cusp objects. Membership of a class does not appear to correlate with the presence of a massive stellar disk.

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arXiv1999

The Hubble Deep Field South - STIS Imaging

We present the imaging observations made with the Space Telescope Imaging Spectrograph of the Hubble Deep Field - South. The field was imaged in 4 bandpasses: a clear CCD bandpass for 156 ksec, a long-pass filter for 22-25 ksec per pixel typical exposure, a near-UV bandpass for 23 ksec, and a far-UV bandpass for 52 ksec. The clear visible image is the deepest observation ever made in the UV-optical wavelength region, reaching a 10 sigma AB magnitude of 29.4 for an object of area 0.2 square arcseconds. The field contains QSO J2233-606, the target of the STIS spectroscopy, and extends 50"x50" for the visible images, and 25"x25" for the ultraviolet images. We present the images, catalog of objects, and galaxy counts obtained in the field.

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arXiv1999

The Lyman-alpha forest of the QSO in the Hubble Deep Field South

The quasar in the Hubble Deep Field South (HDFS), J2233-606 (z=2.23) has been exhaustively observed by ground based telescopes and by the STIS spectrograph on board the Hubble Space Telescope at low, medium and high resolution in the spectral interval from 1120 A to 10000 A. This very large base-line represents a unique opportunity to study in detail the distribution of clouds associated with emitting structures in the field of the quasar and in nearby fields already observed as part of the HDFS campaign. Here we report the main properties of the Lyman-alpha clouds in the intermediate redshift range 1.20-2.20, where our present knowledge has been complicated by the difficulty in producing good data. The number density is shown to be higher than what is expected by extrapolating the results from both lower and higher redshifts: 63\pm8 lines with log N_{HI}\geq14.0 are found (including metal systems) at =1.7, to be compared with ~40 lines predicted by extrapolating from previous studies. The redshift distribution of the Lyman-alpha clouds shows a region spanning z=1.383-1.460 (comoving size of 94 h^{-1}_{65} Mpc, Omega_o=1) with a low density of absorption lines; we detect 5 lines in this region, compared with the 16 expected from an average density along the line of sight. The two point correlation function shows a positive signal up to scales of about 3 h^{-1}_{65} Mpc and an amplitude that is larger for larger HI column densities. The average Doppler parameter is about 27 km/s, comparable to the mean value found at z > 3, thus casting doubts on the temperature evolution of the Lyman-alpha clouds.

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arXiv1998

VLT and HST observations of a candidate high redshift elliptical galaxy in the Hubble Deep Field South

The combined use of the ESO Very Large Telescope (VLT) UT1 Science Verification (SV) images and of the Hubble Space Telescope (HST) Hubble Deep Field South observations allows us to strengthen the identification as a candidate elliptical galaxy of the Extremely Red Object HDFS 223251-603910 previously identified by us on the basis of NICMOS and Cerro Tololo Interamerican Observatory imaging. The photometry presented here includes VLT data in U, B, V, R, I, a STIS unfiltered image, NICMOS J, H, and K band data, thus combining the 16.5 hours of VLT SV exposures with 101 hours of HST observing. The object is detected in all images except the VLT U band and is one of the reddest known with B-K=9.7+-0.5. We consider a wide range of models with different ages, metallicities, star formation histories and dust content, and conclude that the observed spectral energy distribution agrees best with that of an old elliptical galaxy at redshift just below 2. Alternative possibilities are discussed in light of their likelihood and of the perspective of spectroscopic confirmation.

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arXiv1998

The Specific Globular Cluster Frequencies of Dwarf Elliptical Galaxies from the Hubble Space Telescope

The specific globular cluster frequencies (S_N) for 24 dwarf elliptical (dE) galaxies in the Virgo and Fornax Clusters and the Leo Group imaged with the Hubble Space Telescope are presented. Combining all available data, we find that for nucleated dEs --- which are spatially distributed like giant ellipticals in galaxy clusters --- S_N(dE,N)=6.5 +- 1.2 and S_N increases with M_V, while for non-nucleated dEs --- which are distributed like late-type galaxies --- S_N(dE,noN)=3.1 +- 0.5 and there is little or no trend with M_V. The S_N values for dE galaxies are thus on average significantly higher than those for late-type galaxies, which have S_N < 1. This suggests that dE galaxies are more akin to giant Es than to late-type galaxies. If there are dormant or stripped irregulars hiding among the dE population, they are likely to be among the non-nucleated dEs. Furthermore, the similarities in the properties of the globular clusters and in the spatial distributions of dE,Ns and giant Es suggest that neither galaxy mass or galaxy metallicity is responsible for high values of S_N. Instead, most metal-poor GCs may have formed in dwarf-sized fragments that merged into larger galaxies.

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arXiv1998

An Extremely Red r^{1/4} Galaxy in the Test Image of the Hubble Deep Field South

We report the serendipitous discovery of an extremely red object in the Hubble Deep Field South (HDFS) Test NICMOS (Near Infrared Camera and Multi Object Spectrograph) field of view. The object is resolved in the NICMOS image and has a light profile very well described by an r^{1/4} law with effective radius r_e=0."20+-0."05 and H_{AB}=21.7+-0.1 magnitudes. In contrast, the galaxy is undetected in the R and I band ground based images taken at the CTIO 4 m Blanco Telescope, giving a lower limit to the color of (R-H)_{AB}>3.9 and (I-H)_{AB}>3.5 at the 95% confidence level. The colors of a range of synthetic galactic spectra are computed, showing that the object is likely to be an ``old'' elliptical galaxy at redshift z>~1.7. Alternatively the colors can be reproduced by an ``old'' elliptical galaxy at somewhat lower redshift (z>~1) with significant amount of dust, or by a younger galaxy at higher redshift. This object represents a very interesting target for future VLT observations.

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arXiv1998

Globular Clusters in Dwarf Elliptical Galaxies

Globular clusters (GCs) provide one way of determining when the most important periods of star formation in dwarf elliptical (dE) galaxies occurred. Thus, they are also useful for comparing the star formation histories of dwarf and giant galaxies. We present results on the GC systems of 24 nearby dE galaxies imaged with the Hubble Space Telescope. We find that the specific GC frequency, S_N, increases with M_V. Also, S_N in dEs is more like the values found in giant ellipticals rather than in spirals, and the mean S_N for nucleated dEs is roughly a factor of 2 higher than for non-nucleated dEs. The colors suggest that most of the GCs formed before the bulk of the stars in the galaxies.

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arXiv1998

Color--Luminosity Relations for the Resolved Hot Stellar Populations in the Centers of M 31 and M 32

We present Faint Object Camera (FOC) ultraviolet images of the central 14x14'' of Messier 31 and Messier 32. The hot stellar population detected in the composite UV spectra of these nearby galaxies is partially resolved into individual stars, and their individual colors and apparent magnitudes are measured. We detect 433 stars in M 31 and 138 stars in M 32, down to detection limits of m_F275W = 25.5 mag and m_F175W = 24.5 mag. We investigate the luminosity functions of the sources, their spatial distribution, their color-magnitude diagrams, and their total integrated far-UV flux. Although M 32 has a weaker UV upturn than M 31, the luminosity functions and color-magnitude diagrams of M 31 and M 32 are surprisingly similar, and are inconsistent with a majority contribution from any of the following: PAGB stars more massive than 0.56 Msun, main sequence stars, or blue stragglers. Both the the luminosity functions and color-magnitude diagrams are consistent with a dominant population of stars that have evolved from the extreme horizontal branch (EHB) along tracks with masses between 0.47 and 0.53 Msun. These stars are well below the detection limits of our images while on the zero-age EHB, but become detectable while in the more luminous (but shorter) AGB-Manque' and post-early asymptotic giant branch (PEAGB) phases. The FOC observations require that only a only a very small fraction of the main sequence population (2% in M 31 and 0.5% in M 32) in these two galaxies evolve though the EHB and post-EHB phases, with the remainder evolving through bright PAGB evolution that is so rapid that few if any stars are expected in the small field of view covered by the FOC.

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arXiv1998

High Redshift Galaxies in the Hubble Deep Field: II. Colours and Number Counts

We discuss the deep galaxy counts from the Hubble Deep Field (HDF) imaging survey. At faint magnitudes, the slope of the differential number-magnitude relation is flatter than 0.2 in all four HDF bandpasses. In the ultraviolet, a fluctuation analysis shows that the flattening observed below U=26 mag is not due to incompleteness and is more pronounced than in the other bands, consistent with the idea that a redshift limit has been reached in the galaxy distribution. A reddening trend of about 0.5 magnitude is observed at faint fluxes in the colour-magnitude diagram, (U-V)_{eff} vs. V. We interpret these results as the effect of intergalactic attenuation on distant galaxies. At flux levels of AB=27 mag and in agreement with the fluctuation analysis and the colour-magnitude relation, about 7% of the sources in U, 30% in B and 35% in V are Lyman-break ``dropouts'', i.e. candidate star-forming galaxies at z>2. By integrating the number counts to the limits of the HDF survey we find that the mean surface brightness of the extragalactic sky is dominated by galaxies that are relatively bright and are known to have =0.6. To AB=29 mag, the integrated light from resolved galaxies in the I-band is 2.1\pm 0.4 \times 10^{-20} erg/cm^2/s/Hz/sr, and its spectrum is well described by a broken power-law. We discuss the predictions for the counts, colours, and luminosity densities from standard low-q_0 pure-luminosity-evolution models without dust obscuration, and find that they are unable to reproduce all the observed properties of faint field galaxies.

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arXiv1997

The STIS Parallel Survey: Introduction and First Results

The installation of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) allows for the first time two-dimensional optical and ultraviolet slitless spectroscopy of faint objects from space. The STIS Parallel Survey (SPS) routinely obtains broad band images and slitless spectra of random fields in parallel with HST observations using other instruments. The SPS is designed to study a wide variety of astrophysical phenomena, including the rate of star formation in galaxies at intermediate to high redshift through the detection of emission-line galaxies. We present the first results of the SPS, which demonstrate the capability of STIS slitless spectroscopy to detect and identify high-redshift galaxies.

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arXiv1997

Hot Evolved Stars in the Centers of M 31 and M 32

We present UV images of M 31 and M 32, as observed by HST with the refurbished FOC. The galaxies were observed through the F175W and F275W filters, allowing the construction of color magnitude diagrams (CMDs) for the hundreds of detected sources found in each image. Comparison of these data with the stellar evolutionary tracks of horizontal branch stars and their progeny shows that for the first time outside of our own Galaxy, we may be measuring the colors of individual stars that are evolving along post asymptotic giant branch (PAGB), post-early AGB, and AGB-Manque' paths. Searching to the 6 sigma detection limit, we find 1349 stars in M 31 and 183 stars in M 32. We compare the distribution of stars in the CMDs with the expectations from theory.

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arXiv1997

A Far Ultraviolet Analysis of the Stellar Populations in Six Elliptical and S0 Galaxies

We have analyzed the far-ultraviolet (FUV) spectra of six elliptical and S0 galaxies in order to characterize their hot stellar populations. The spectra were obtained using the Hopkins Ultraviolet Telescope (HUT) in March 1995. These data, together with the spectra of two galaxies observed with HUT in 1990, represent the only FUV spectra of early type galaxies that extend to the Lyman limit at 912 A and therefore include the "turnover" in the spectral energy distribution below Lyman alpha. Using an extensive new grid of synthetic spectra which match the HUT resolution and cover the relevant parameter space of temperature and gravity, we have constructed synthetic spectral energy distributions by integrating over predicted stellar evolutionary tracks for horizontal branch stars and their progeny. When the models are compared with the HUT data, we find that those with supersolar metal abundances and helium best reproduce the flux across the entire HUT wavelength range, while those with subsolar Z & Y fit less well, partly because of a significant flux deficit shortward of 970 A in the models. We find that AGB-Manque evolution is required in all fits to the HUT spectra, suggesting that all of the galaxies have some subdwarf B star population. At any Z & Y, the models that best match the HUT flux are dominated by stars evolving from a narrow range of envelope mass on the blue end of the horizontal branch.

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arXiv1996

Far-UV Line Strengths in Elliptical Galaxies

Much of the far-UV emission from elliptical galaxies is thought to arise from extreme horizontal branch stars and related objects. Only about 10% of the stellar population needs to evolve through this phase even in galaxies with the strongest UV upturn. However it is not yet clear if this population represents the extreme low-metallicity or high-metallicty tail of the distribution, or rather arises from the overall population through some metallicity-insensitive mechanism that causes increased mass loss in a small fraction of RGB stars. We investigate the utility of far-UV line strengths for deciding between these possiblities. Complications include the fact that the line strengths reflect both the temperature distribution and the metallicity distribution of the stars, that there may be abundance anomalies introduced on the RGB, and that metals are likely to be redistributed by gravitational settling and radiative diffusion in the atmospheres of hot high-gravity stars. Line-strength measurements from Astro-2 HUT spectra are considered in this context.

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arXiv1995

Determination of the Hubble constant from observations of Cepheid variables in the galaxy M96

New Hubble Space Telescope observations of Cepheid variable stars in the nearby galaxy M96 give a distance to the host galaxy group, Leo-I, of 11.6+/-0.8 Mpc. This value, used in conjunction with several reliable secondary indicators of relative distance, constrains the distances to more remote galaxy clusters, and yields a value of the Hubble constant (Ho=69+/-8 km/s/Mpc) that is independent of the velocity of the Leo-I group itself.

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